In men, testosterone secretion affects neurobehavioral functions such as sexual arousal, aggression, emotional tone, and cognition. Beginning at approximately age 50, men secrete progressively lower amounts of testosterone; about 20% of men over age 60 have lower-than-normal levels. The psychiatric sequelae are poorly understood, yet there is evidence of an association with depressive symptoms. The authors reviewed 1) the physiology of the hypothalamic-pituitary-gonadal axis and its changes with age in men; and 2) the evidence linking testosterone level and major depression in men. Data on this relationship are derived from two types of studies: observational studies comparing testosterone levels and secretory patterns in depressed and non-depressed men, and treatment studies using exogenous androgens for male depression. The data suggest that some depressed older men may have state-dependent low testosterone levels and that some depressed men may improve with androgen treatment.